The present invention relates to novel S-(4-biphenyl)-thiosulphuric acids and salts thereof, to a process for their preparation starting from S-(4-biphenyl)-sulphinic acids and salts thereof and the preparation of 4-mercaptobiphenyls from the S-(4-biphenyl)-thiosulphuric acids and their salts.
4-Mercaptobiphenyls are important intermediates for preparing pharmaceutically and agiochemilcally active compounds (see, for example, BE-A 887 423, U.S. Pat. No. 3,912,757 and WO 96/25 936). Some processes for preparing 4-mercaptobiphenyls are already known; however, all of them are unsatisfactory.
Thus, biphenylsulphonyl chloride can be reduced using amalgamated zinc, metallic tin or tin(II) chloride (see, for example, J.A.C.S. 66, 1674 (1944), Chem. Ber. 13, 386 (1880) and Ann. Univ. Marie Curie-Sklodowska, Section Aa, Volume Date 1966 No. 21, 65 to 83 (1967)). In all of these processes, waste waters containing heavy metal salts are produced, the disposal of which involves great costs.
It is also possible to diazotize 4-aminodiphenyl, followed by reaction with potassium ethyl xanthate and hydrolysis of the resulting thioester (see DE-A 23 17 142, pp. 34 to 35). Here, water-soluble nickel chloride has to be employed which likewise passes into the waste water, where the disposal involves high costs.
The reaction of p-hydroxybiphenyl with dimethylthiocarbamoyl chloride followed by Newman-Kwart rearrangement and finally hydrolysis (see J. Het. Chem. 15, 281 (1978) and WO 96/25 936) affords 4-mercaptobiphenyl only in a yield of 39%.
When 4-bromobiphenyl is reacted with sodium methyl sulphide or sodium ethyl sulphide and the resulting thioether is cleaved, good yields of 4-mercaptobiphenyl (for example 96% of theory) are only obtained when handling of the carcinogenic hexamethylphosphoric triamide is accepted (see Tetrahedron Lett. 21, 3099 (1980)). The use of other solvents, for example dimethylformamide, results in considerably lower yields of 4-mercaptobiphenyl (for example 67% of theory--see Synthesis 9, 751 (1983)).
The reaction of 4-bromobiphenyl with elemental sulphur and the cleavage of the reaction product with lithium aluminium hydride to give 4-mercaptobiphenyl requires the use of lithium aluminium hydride, which is difficult to handle (see Tetrahedron Lett. 13, 1283 (1972).
Also known are reduction processes for preparing 4-mercaptobiphenyl starting from aromatic sulphonyl chlorides and aromatic disulphides. However, reduction with hydrogen and noble metal catalysts requires temperatures of up to 150.degree. C. and pressures of up to 150 bar (see EP-A 2755), and reduction with red phosphorus and iodine necessarily leads to phosphoric acid and hydrogen chloride being produced (see Chem. Ber. 99, 375 (1966)) and requires red phosphorus, which is difficult to handle.
Finally, it is known that Bunte salts, i.e. salts of the type R-S-SO3M (R=organic radical, M=monovalent metal) in aqueous-acidic media generally hydrolyse to give thiols (see Angew. Chem. 79, 525 (1967)). However, in the present case disulphides were produced.